The LED (Light Emitting Diode) chip is a primary component of a light emitting diode and is formed via successively epitaxially growing semiconductor light emitting materials. An LED chip is made of a semiconductor material, such as GaP (Gallium Phosphide), GaAlAs (Gallium Aluminum Arsenide), GaAs (Gallium Arsenide), or GaN (Gallium Nitride), which has a PN junction thereinside and a unidirectional conductivity.
For example, in fabricating a blue light LED, a high-quality GaN-based epitaxial film is grown on a sapphire (Al2O3) substrate. However, the sapphire substrate has poor electric and thermal conductivities. Thus, positive and negative electrodes have to be arranged in the same side in the conventional blue light LED, which decreases the light emitting area. Further, the current crowding effect thereof raises the forward resistance and the forward voltage drop.
To solve the above-mentioned problems, the current solution is to form a new substrate on the GaN-based epitaxial film that has been grown on the sapphire substrate via electroplating a metal film on the GaN-based film or bonding a wafer to the metal film. Then, the sapphire substrate is removed with an LED-laser lift-off method. Thus, the GaN-based epitaxial film adheres to the new substrate via metal bonding. The high thermal conductivity and high electric conductivity of the new substrate makes LED more adaptive to high current applications and solves the heat-dissipation problem occurring in a high luminous flux situation.
Refer to FIG. 1. In a conventional LED-laser lift-off method, an epitaxial layer 20 is grown on a transient substrate 10 (such as a sapphire substrate) firstly; next, separation channels 22 are formed in the epitaxial layer 20 with an etching method to define a plurality of chip sections 21; next, a support substrate 40 having an adhesion metal layer 30 is bonded to the epitaxial layer 20; next, a photomask (not shown in the drawing) with hollowed-out regions (of a circular shape, a rectangular shape, or another shape) is placed near the transient substrate 10, and a laser light 50 passes through the hollowed-out regions of the photomask to illuminate the transient substrate 10. Refer to FIG. 2. Each laser illumination area 51 of the laser light 50 includes a chip section 21 corresponding to one hollowed-out region and the separation channels 22 around the chip section 21. After the laser light 50 carpet-scans and heats the entire transient substrate 10, the transient substrate 10 is lifted off from the epitaxial layer 20. At this time, the chip sections 21 of the epitaxial layer 20 are bonded to the support substrate 40 via the adhesion metal layer 30.
Refer to FIG. 3. When the transient substrate 10 having the epitaxial layer 20 is boned to the support substrate 40, there are bonding-induced warpages appearing in the perimeter thereof, and the warpages will cause alignment problem. When the laser light 50 projects through the hollowed-out regions to illuminate the transient substrate 10, the laser illumination areas 51 for the chip sections 21 in the perimeter are apt to shift toward the center. Even though the hollowed-out regions have been precisely defined in the photomask, the separation channels 22 between neighboring chip sections 21 in the perimeter of the transient substrate 10 still will be illuminated and heated twice when the laser light 50 carpet-scans the entire transient substrate 10. Thus, the adhesion metal layer 30 on the above-mentioned separation channel is also heated twice, which will results in high temperature and damage the adhesion metal layer 30. The current solution is widening the separation channels 22 to prevent the adhesion metal layer 30 of the separation channel 22 from being heated twice.
Refer to FIG. 4. When the laser light 50 illuminates each chip section 21, the outward stress of the chip section 21 will affect the neighboring chip sections 21. When the laser light 50 carpet-scans the entire transient substrate 10, each chip section 21 of the epitaxial layer 20 may be illuminated by the laser light 50 several times, and the induced stress F1 may damage the structure of LED.
An U.S. Pat. No. 7,202,141 disclosed a lift-off method. In the prior art, a gallium arsenide layer is formed on a single-crystal aluminum oxide substrate. Channels are formed on the gallium arsenide with a reaction ion etching method or another method to symmetrically divide the gallium arsenide layer into a plurality of identical gallium arsenide sections. A metal baseplate is formed on the gallium arsenide sections by the electroplated fabrication. An ultraviolet laser is used to cut the metal baseplate exactly above the regions between each two adjacent gallium arsenide sections. A support film is formed on the metal baseplate. Then, a laser lift-off process is performed to remove the substrate and the metal baseplate, whereby gallium arsenide chips are formed on the support film. In the prior art, the channels separate the gallium arsenide sections and greatly reduce the stress concentration occurring in the single-crystal interfaces of the aluminum oxide and the gallium arsenide. Thus, the stress-induced damage is less likely to occur.
An U.S. Pat. No. 6,617,261 disclosed a method for forming a channel-patterned gallium nitride layer (GaN) on a single-crystal aluminum oxide substrate and a structure fabricated by the same method. Firstly, a gallium nitride layer (GaN) is formed on a single-crystal aluminum oxide substrate via a gallium nitride nucleation layer. Next, a silicon oxide layer is deposited on the upper surface of the gallium nitride layer, and a plurality of strip-shaped patterns are photolithographically formed on the silicon oxide layer. Next, grooves corresponding to the strip-shaped patterns are formed on the gallium nitride layer with a dry or wet etching method, wherein the width of the groove is between 100 Å and 1 μm.
In the prior arts mentioned above, channels are formed between the gallium arsenide sections. The channels of the prior arts can only solve the problem of the stress concentration occurring in the interface between the gallium arsenide and the substrate during a laser lift-off process. However, the prior arts cannot solve the problem that the metal baseplate is damaged by the thermal stress occurring during repeated heating.